How to get world back?
Blender's concept is to work with objects. So if you don't see your world in the outliner anymore, you probably just unlinked it. The object itself still exists in the background and will be kept until you close blender.
In the gif you see how to might have unlinked the world and how you get the (still existing but not linked) world back.

If you closed blender anyway and the unlinked map is lost, just create a new world by hitting + New in the world tab.